Current biological invasive and noninvasive pressure monitoring systems reference measured pressures to a fixed point of reference, i.e., physical height on the patient's body. The hydrostatic pressure of the atmosphere at that point or reference height then becomes the zero for the system. Pressure impulses in such systems, when conveyed via a fluid conduit to the point of conversion to electrical signal, i.e., the pressure transducer; are usually conveyed to the transducer(s) via a non-compliant flexible tubing means containing a saline solution compatible with blood (e. g. 0.9% in NaCl) and an anticoagulant like heparin sodium which prevents clotting where conveyance fluid interfaces with patient blood, usually in a catheter.
Such transducers when mounted at the patient's bedside, receive via the fluid conduit, an accurate representation of the pressure at the point of measurement (e.g. pulmonary artery, right heart chambers, radial artery, femoral artery, cerebrospinal fluid, et. al.); but only when the monitoring system receiving electrical impulses from the transducers is zeroed to atmospheric pressure at the reference point on the patient's body and only when the transducer(s) are horizontally level with the reference point. When transducer(s) move from this level position, a significant hydrostatic artifact is introduced which corresponds to the positive or negative artifactual pressure exerted on the system by the weight of the effective fluid column of pressure generated within the conveyance conduit as a function of the vertical height difference between proximal and distal ends of the conduit, in this case, the source of pressure on the patient and the transducer.
In order that the pressure transducer(s) do not experience such artifactual error from these vertical height disparities, the transducer(s) are usually mounted in a fixed position that corresponds to the horizontal height of the chosen reference point on the patient (e.g. estimate of right atrial height in cardiovascular measurements, estimate of brain height in neurosurgical patients). This maneuver is known as leveling and it provides for a net zero hydrostatic artifact that might be produced by height differences between patient and transducer(s). Once at level position, the transducer(s) are then zeroed, i.e., exposed to atmospheric pressure by opening the previously closed conduit system to ambient air via a stopcock or other venting device. This zeroing is important because it allows the monitoring system that receives the electrical signal(s) from the transducer(s) to adjust for the inherent zero-offset present in the individual transducers.
Self leveling systems in use utilize a reference probe placed on the patient which is connected via a fluid conduit system either to a differential transducer to provide mechanical subtraction of the resultant hydrostatic height difference between patient reference point and transducer; or to an additional reference transducer mounted at the same height with the pressure measurement transducer(s) which sends electrical signal(s) to a subtraction circuit to provide electrical analog and(or) digital subtraction and hence, correction for any such hydrostatic height differences. Such systems, because they utilize multiple transducers (at least one for height correction and reference), place a significant drain on the amperage required to maintain a constant excitation or input voltage to the transducer setup, and hence to date, can only be used on one specific monitor type whose circuitry has been modified to either adjust to the increased load or to adjust to the reduced resulting output from the transducers in an analog subtraction arrangement.
Biological pressure transduction systems currently utilize multiple methods to transmit pressure signals and convert them into electrical signals which are then delivered to one or several monitoring means and(or) data recording and(or) data storage systems (e.g. U.S. Pat. Nos. 3,946,724, 4,899,760, 4,890,630, 3,095,872, and 4,779,626) for display and data use.
The method described by J P Blackburn in Br. Med. J.;V.4f825,12/68, provides a means for constant reference between a biological pressure transducer and a patient reference probe. A closed system similar to this is described in U.S. Pat. No. 4,779,626. This system along with that described in U.S. Pat. No. 4,576,035 utilize opposing double-input differential pressure transducers for this purpose. Most recently, U.S. Pat. No. 5,103,832 provides for the use of a single-input non differential pressure transducer by providing a reference probe and subtraction circuitry. Though the systems described provide for more constant external reference to correct for hydrostatic height differences, even those with a separate reference transducer require that the transducer be mounted at an identical height as a single or multiple biological pressure transducers. In any of these systems that use multiple transducers for a single monitoring channel, the circuit characteristics of the monitoring device that supplies input excitation voltage to each monitoring channel must be altered to handle the increased load of additional transducer components without changing the voltage to pressure slope characteristics which are fixed by industry standards.